Press forming is a process of transferring a shape of a tool to a target press-forming material (metal sheet) by pressing the tool to the press-forming material. In press forming, after a press-formed material removed from the tool (after release from a tool) springback (elastic deformation) arises and the press-formed material has often an undesirable shape.
Such springback is known to be caused by residual stress in a press-formed material before release from the tool. Conventionally, a numerical analysis method such as a finite element method has been used to estimate the shape after springback and to analyze the cause of the springback.
A conventional analysis of factors contributing to the springback includes a “press-forming analysis method” disclosed in Patent Literature 1. The “press-forming analysis method” disclosed in Patent Literature 1 includes the following three processes.
Process 1 includes calculating data of, for example, a shape of a press-formed material before release from a tool. By the process 1, it is performed, based on the data before release from the tool, to calculate data of a shape of the press-formed material after release from the tool and to calculate a defined quantity relating to springback.
Process 2 includes adding a modification to a distribution of residual stress in a specific region of the press-formed material before release from the tool. By the process 2, it is performed, based on the modified data, to calculate data of, for example, a shape of the press-formed material after release from the tool, and to calculate a defined quantity relating to the springback after the modification in the distribution of the residual stress in the specific region.
Process 3 includes calculating how the defined quantity is changed by the modification to the distribution of the residual stress in the specific region.
The “press-forming analysis method” in Patent Literature 1 quickly and accurately estimates an effect of residual stress on springback in any region of a press-formed material after press forming (before release from a tool) for consideration of countermeasures for springback.
In conventional methods of analyzing springback, as represented by Patent Literature 1, the press forming discussed therein is cold press forming in which a press-forming material is not heated.
In recent years, a high strength steel sheet has been more widely used as a steel sheet for automotive parts to improve both fuel efficiency and collision safety performance.
Since the high strength steel sheet has high flow stress to deformation, service life of the tool is shortened if the high strength steel sheet is processed by cold press forming. The high strength steel sheet can be processed by only limited processes such as deep-drawing and high stretch-flanging, which are not high-pressure processes.
To avoid such problems, “warm press forming” that performs press forming after a press-forming material is heated to a predetermined temperature is used for the high strength steel sheet. Warm press forming is performed at a higher temperature than in cold press forming, and decrease the flow stress of the high strength steel sheet to improve deformability. This technique prevents defects such as cracks from occurring in press forming. Such a technique of the warm press forming is disclosed in Patent Literature 2, for example.